1. Technical Field
The present invention relates, in general, to radiosensitization and, in particular, to a method of potentiating radiation therapy using Co(III) complexes of water soluble, synthetic porphyrins.
2. Background Information
The use of porphyrin derivatives in combination with visible light as a treatment for cancer is well established. The rationale behind this treatment is that certain porphyrins accumulate in neoplastic tissue; when these porphyrins are exposed to visible light, highly reactive chemical species are formed that destroy the tumor.
One drawback to this "photodynamic therapy" is that deep seated tumors in internal organs are difficult to irradiate with visible light. This problem can be overcome, however, by the use of ionizing radiation, which penetrates tissues more readily than visible light, in conjunction with porphyrins that potentiate the effects of ionizing radiation on tumors.
Several attempts have been made to use porphyrins and metalloporphyrins as radiosensitizers. Hematoporphyrin has been shown to potentiate the effectiveness of X-rays in treating rhabdomyosarcoma in mice. While little or no beneficial effect has been seen in human patients administered hematoporphyrin, metal complexes of meso-tetra(p-carboxyphenyl)porphine (particularly the Zn and Ni complexes) have been found to be radiosensitizers of human tumor cells in vitro
Most of the radiosensitizers described in the literature sensitize only hypoxic (oxygen deficient) cells. Since some tumors are thought to have regions of hypoxic tissue which are insensitive to the effects of ionizing radiation, these radiosensitizers are therapeutically beneficial because they potentiate the radiation killing of hypoxic cells but do not sensitize normal tissue which is well oxygenated. These compounds, however, are not useful in sensitizing oxic (well oxygenated) tumor cells.